1. Field of the Invention
The present invention relates to a speed change control method for an automatic transmission for vehicles, and more particularly, to a speed change control method whereby an optimum gearshift position is automatically selected by a fuzzy inference in accordance with road conditions, vehicle driving conditions, the driving intention of a driver, etc. when driving on flat roads such as urban district streets, and winding or straight ascending slope roads and the like in mountainous areas.
2. Description of the Related Arts
In a conventional automatic transmission for a vehicle, shift patterns are preset in accordance with opening of a throttle (engine load) and vehicle speed, and a gearshift position is set using the shift patterns according to the opening of the throttle and the vehicle speed that are detected, thus automatically shifting the gear. The conventional automatic speed change control method presents no particularly serious problem and ensures smooth speed change with no difficulty as long as the gear is shifted while driving on flat roads such as urban district streets. However, driving in mountainous areas encounters straight ascending slope roads, ascending slope roads with many curves, descending slope roads that require strong engine braking, and gentle, long descending slope roads. Also, some drivers let their vehicles accelerate rapidly on descending slopes and step on brake pedals deeply immediately before cornering. During such driving in mountainous areas, it is rather difficult to select an optimum gearshift position in accordance with vehicle driving conditions, driving intention of a driver, road conditions, and the like. For this reason, there has been a need to achieve a method that permits easy driving operation, good car driving performance, and more comfortable driving even when driving in mountainous areas.
In response to the foregoing demand, speed change control methods wherein "fuzzy control" is performed to select an optimum gearshift position in accordance with the aforementioned vehicle driving conditions or the other conditions are known, for instance, by Japanese unexamined patent publications No. S63-246546, No. H02-3738, etc. These conventional speed change control methods are designed to determine all of optimum gearshift positions by a fuzzy inference for driving on urban streets and in mountainous areas. Accordingly, the speed change control methods based on the conventional fuzzy control have such disadvantages as many rules and complicated membership functions, requiring a computer with a large capacity for practical applications. Further, because of the many rules and the complicated membership functions, tuning is difficult, therefore making it difficult to apply them to other models.
In addition, when the speed change control methods based on the fuzzy control are newly adopted, they are likely to feel foreign to drivers who are accustomed to driving on normal flat roads in urban districts by the conventional automatic speed change control methods. This is because a shift of gear is made in response to subtle changes in driving conditions such as riding over a small bump or a light step on an accelerator, whereas no shift of gear would be made in the conventional methods.
In Japanese unexamined patent publication No. H2-212655 a speed change control method is proposed wherein diverse parameters that represent vehicle driving conditions are detected, fuzzy inference is implemented based on the detected signals of the parameters and preset membership functions to assess the magnitude of the driving resistance, and a speed change map for high-load driving is selected to replace a speed change map for normal driving if the driving resistance value is greater than a predetermined value, thus determining the gearshift positions according to the speed change map for high-load driving. However, according to the proposed speed change control method, the same speed change map is used for both roads of straight ascending slopes and roads of ascending slopes with many curves. Therefore, it does not permit sufficiently accurate speed change control for the foregoing diverse road conditions and driving intentions, etc. that are encountered while driving in mountainous areas.
Further, if the speed change map for normal driving is replaced by the speed change map for high-load driving to set the gearshift position according to the map for high-load driving immediately after it is judged that the driving resistance value is greater than the predetermined value, then the speed change causes an uncomfortable feeling to a driver. This is especially true when the speed in the gearshift position selected by the speed change map for normal driving is lower than that selected by the speed change map for high-load driving.
Furthermore, in the conventional calculation of gradient resistance, a great error is unavoidably involved in a calculated gradient resistance value in the case of a large turn of a steering wheel. In other words, it is disadvantageous in that a gradient resistance calculated while cornering on a descending wound road is smaller than an actual value, while the gradient resistance calculated while cornering on a flat road is larger than the actual value, causing the system to erroneously presume that the vehicle is climbing a slope. Moreover, it is difficult to perform accurate calculation of the gradient resistance due to noises detected. The calculation error increases especially when a vehicle climbs a steep gradient immediately after turning a hairpin curve following a flat road.